Nonlinear Acoustic Effects Research Articles

Nonlinear saturation effects in ultrasound fields of diagnostic-type transducers used for kidney stone propulsion

A novel therapeutic application of ultrasound for repositioning kidney stones is being developed. The method uses acoustic radiation force to expel mm-sized stones or to dislodge even larger obstructing stones. A standard diagnostic 2.3 MHz C5-2 array probe has been used to generate pushing acoustic pulses. The probe comprises 128 elements equally spaced at the 55 mm long convex cylindrical surface with 41.2 mm radius of curvature. The efficacy of the treatment can be increased by using higher transducer output to provide stronger pushing force; however, nonlinear acoustic saturation effect can be a limiting factor. In this work, nonlinear propagation effects were analyzed for the C5-2 transducer using a combined measurement and modeling approach. Simulations were based on the 3D Westervelt equation; the boundary condition was set to match low power measurements. Focal waveforms simulated for several output power levels were compared with the fiber-optic hydrophone measurements and were found in good agreement. It was shown that saturation effects do limit the acoustic pressure in the focal region of the transducer. This work has application to standard diagnostic probes and imaging. [Work supported by RSF 14-12-00974, NIH EB007643, DK43881 and DK092197, and NSBRI through NASA NCC 9-58.]

The Use of Nonlinear Acoustics as an Energy-Efficient Technique for Aerosol Removal

This article is a feasibility study on using nonlinear acoustic effects, acoustic streaming and acoustic radiation pressure, for aerosol removal in an air duct. Unlike previous research, which used acoustics solely to cause aerosol agglomeration prior to aerosol removal in traditional duct collection systems, this article considers the acoustic streaming effect, which is significant but was previously neglected. Monodispersed polystyrene spheres with diameters ranging from 0.3 to 6 μm were tested. The proposed system removed 12–20% of the submicron aerosols and 25–32% of the micron aerosols when the airflow rate was approximately 90 L/min. Acoustic streaming introduces stagnation points on the surface of the air duct and removes the aerosols by deposition. Acoustic radiation pressure causes aerosols to form agglomerates. This enhances inertial impaction and/or gravitational sedimentation, which further enhances the removal efficiency of micron aerosols. The particle-removal efficiency is proportional to the duration that the aerosols are exposed to the acoustic field. The pressure drop due to the nonlinear acoustic effects is negligible; thus, power consumption is minimal. This system has the potential to be developed into an energy-efficient technique for aerosol removal.Copyright 2014 American Association for Aerosol Research

Experimental observation of acoustic sub-harmonic diffraction by a grating

A diffraction grating is a spatial filter causing sound waves or optical waves to reflect in directions determined by the frequency of the waves and the period of the grating. The classical grating equation is the governing principle that has successfully described the diffraction phenomena caused by gratings. However, in this work, we show experimental observation of the so-called sub-harmonic diffraction in acoustics that cannot be explained by the classical grating equation. Experiments indicate two physical phenomena causing the effect: internal scattering effects within the corrugation causing a phase shift and nonlinear acoustic effects generating new frequencies. This discovery expands our current understanding of the diffraction phenomenon, and it also makes it possible to better design spatial diffraction spectra, such as a rainbow effect in optics with a more complicated color spectrum than a traditional rainbow. The discovery reveals also a possibly new technique to study nonlinear acoustics by exploitation of the natural spatial filtering effect inherent to an acoustic diffraction grating.

Ultrasonic modulated waves for diagnostics of concrete elements in bending

The paper presents the experimental study on the diagnostics of concrete elements with the use of nonlinear acoustic effects. The tests were conducted on a concrete plate subjected to ultrasonic waves modulated with and without an additional low frequency actuator. The experimental results showed that the new method based on the direct modulation of diagnostic ultrasonic waves also provided sub-harmonic frequencies that indicated the presence of damage. The new method does not require a low frequency actuator for generation of low frequency oscillations, and therefore, is more suitable for practical application.

Nonlinear acoustic properties of ex vivo bovine liver and the effects of temperature and denaturation

Thermal ablation by high intensity focused ultrasound (HIFU) has a great potential for the non-invasive treatment of solid tumours. Due to the high pressure amplitudes involved, nonlinear acoustic effects must be understood and the relevant medium property is the parameter of nonlinearity B/A. Here, B/A was measured in ex vivo bovine liver, over a heating/cooling cycle replicating temperatures reached during HIFU ablation, adapting a finite amplitude insertion technique, which also allowed for measurement of sound-speed and attenuation. The method measures the nonlinear progression of a plane wave through liver and B/A was chosen so that numerical simulations matched the measured waveforms. To create plane-wave conditions, sinusoidal bursts were transmitted by a 100 mm diameter 1.125 MHz unfocused transducer and measured using a 15 mm diameter 2.25 MHz broadband transducer in the near field. Attenuation and sound-speed were calculated using a reflected pulse from the smaller transducer using the larger transducer as the reflecting interface. Results showed that attenuation initially decreased with heating then increased after denaturation, the sound-speed initially increased with temperature and then decreased, and B/A showed an increase with temperature but no significant post-heating change. The B/A data disagree with other reports that show a significant change and we suggest that any nonlinear enhancement in the received ultrasound signal post-treatment is likely due to acoustic cavitation rather than changes in tissue nonlinearity.

Numerical simulation of nonlinear Lamb waves used in a thin plate for detecting buried micro-cracks.

Compared with conventional linear ultrasonic inspection methods, which are sensitive only to severe defects, nonlinear ultrasonic inspection methods are better for revealing micro-cracks in thin plates. However, most nonlinear ultrasonic inspection methods have only been experimentally investigated using bulk or Rayleigh waves. Numerical studies, especially numerical simulations of Lamb ultrasonic waves, have seldom been reported. In this paper, the interaction between nonlinear S0 mode Lamb waves and micro-cracks of various lengths and widths buried in a thin metallic plate was simulated using the finite element method (FEM). The numerical results indicate that after interacting with a micro-crack, a new wave-packet was generated in addition to the S0 mode wave-packet. The second harmonics of the S0 mode Lamb waves and the new wave-packet were caused by nonlinear acoustic effects at the micro-crack. An amplitude ratio indicator is thus proposed for the early detection of buried micro-cracks.

Nonlinear acoustic waves in media with hysteresis and long-time relaxation

It is known that the media with complex structures (e.g., rocks and ceramics) possess an anomalously strong elastic nonlinearity. Nonlinear acoustic effects are used in a number of important applications, such as seismic waves (earthquakes), non-destructive testing, and contact physics. Two specific features were registered, in most of the experiments: hysteresis in the stress-strain relation and longtime relaxation (slow time). Some physical models, mostly phenomenological, have been suggested to explain these phenomena. However, there are very few works considering the effect of medium hysteresis and relaxation on wave propagation and oscillations in resonators. This presentation is a review of both published and new results in this area. Among the problems discussed are: (1) A brief overview of experiments and models; (2) Analysis of wave propagation in simplified models of media with hysteresis as opposed to the nonlinear waves in media characterized by elastic constants of second and third order; (3) Theory of wave interaction in resonators with hysteresis; (4) Study of waves in media with slow time relaxation; (5) Some data from laboratory and field experiments; (6) Some unsolved issues and the relevant future work will be outlined in conclusion.

Slow Relaxation and Aging Phenomena at the Nanoscale in Granular Materials

Granular matter exhibits a rich variety of dynamic behaviors, for which the role of thermal fluctuations is usually ignored. Here we show that thermal fluctuations can pronouncedly affect contacting nanoscale asperities at grain interfaces and brightly manifest themselves through the influence on nonlinear-acoustic effects. The proposed mechanism based on intrinsic bistability of nanoscale contacts comprises a wealth of slow-dynamics regimes including slow relaxations and aging as universal properties of a wide class of systems with metastable states.

Large eddy simulation of crackle noise in hot supersonic jets

Crackle noise from heated, supersonic jets is investigated through high-fidelity large eddy simulation (LES). Simulations of a military-style nozzle reveal that N-shaped waves responsible for crackle noise emerge directly from the supersonic jet turbulence, and thus do not depend solely on nonlinear acoustic propagation effects. Conditional averaging using a backtracking algorithm furthermore reveals intermittent large-scale flow structures embedded in the jet turbulence as a likely source of the N-shaped waves. The skewness of pressure (Ffowcs Williams et al., 1975) and the skewness of the time derivative of pressure (Gee et al., 2007) are evaluated from full-field simulation data, and assessed as metrics of crackle. We test the hypothesis that the crackle level depends solely on the jet velocity (Ffowcs Williams et al., 1975) by comparing three simulations sharing the same geometry, but having different operating conditions so that the velocity and temperature are varied independently.

Possibilities of tomography system prototype using third-order acoustic nonlinear effects

Third-order nonlinear acoustical tomography is very important for medical diagnostics, because it will provide information on the new and unexplored quantity—third-order nonlinear acoustical parameter. A prototype of the tomography system for reconstructing the distributions of the acoustic nonlinear parameters is developed in our group on the basis of effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected signal at combination frequencies makes it possible to reconstruct the complete image of an object using three transmitters and one receiver. Third-order effects are born from two competing processes—the pure third order interaction (informational for diagnostic purposes) and the twofold interaction of the second order (interfering). To explore the application boundaries of the third-order nonlinear acoustical tomography, these two competing processes are considered in details. Two cases are compared. In the first case, the interaction takes place between two broadband coded signals and one monochromatic signal. In the second case, all three primary signals are broadband coded; then, amplitude of the interfering part of the signal is smaller. Moreover, in this case, the possibility of reconstructing the spatial distribution of the second and third-order nonlinear acoustical parameters arises due to the reciprocity principle generalized on the nonlinear scattering processes.

Detecting closing delaminations in laminated composite plates using nonlinear structural intensity and time reversal mirrors

Closing delaminations in composite laminated structures exhibit a nonlinear dynamic response when excited by high frequency elastic waves. The contact acoustic nonlinear effects taking place at the damage interface act as a mechanism of energy redistribution from the driving frequency to the nonlinear harmonic frequencies. In this paper, we extend the concept of nonlinear structural intensity (NSI) to the analysis of closing delaminations in composite laminated plates. NSI is calculated using a method based on a combination of finite element and finite difference techniques, which is suitable for processing both numerical and experimental data. NSI is proven to be an effective metric to identify the presence and location of closing delaminations. The highly directional nature of orthotropic composites results in vibrational energy propagating in a different direction from that of the initial elastic wave. This aspect reduces the ability to effectively interrogate the damage and, therefore, the sensitivity to the damage. The time reversal mirror technique is explored as a possible approach to overcome the effect of the material directionality and increase the ability to interrogate the damage. Numerical simulations show that this technique is able to overcome the material directionality and to drastically enhance the ability to interrogate the damage.

Diagnostics of concrete plate in bending with ultrasonic waves modulated by low frequency oscillations

The paper presents the experimental study on the inter-modulation method for the diagnostics of concrete elements. The tests were conducted on a concrete plate subjected to ultrasonic waves and low frequency vibrations. The nonlinear acoustic effects, recorded in the experiments, made it possible to detect the presence of damaged zones. Further studies are necessary to establish the relation between the sidebanes of frequency spectra and the size of the damaged zone.

Breaking the acoustic diffraction limit via nonlinear effect and thermal confinement for potential deep-tissue high-resolution imaging

Our recently developed ultrasound-switchable fluorescence (USF) imaging technique showed that it was feasible to conduct high-resolution fluorescence imaging in a centimeter-deep turbid medium. Because the spatial resolution of this technique highly depends on the ultrasound-induced temperature focal size (UTFS), minimization of UTFS becomes important for further improving the spatial resolution USF technique. In this study, we found that UTFS can be significantly reduced below the diffraction-limited acoustic intensity focal size via nonlinear acoustic effects and thermal confinement by appropriately controlling ultrasound power and exposure time, which can be potentially used for deep-tissue high-resolution imaging.

Spectrally resolved detection of mixed acoustic vibrations by photorefractive interferometry

A two-wave mixing photorefractive interferometry method in an anisotropic diffraction scheme, intended to selectively detect an acoustic vibration component at a specific frequency of interest in the presence of strong vibrations at other frequencies, is presented and validated by simulations and experiments. The achieved narrow-band detection is of particular interest for applications where weak vibration signals generated by nonlinear acoustic effects are to be detected with high sensitivity and without cross-talk from the large vibration components from which the acoustic nonlinearity is originating.

A prototype for a tomography system using third-order acoustic nonlinear effects

A prototype of a tomography system for reconstructing the distributions of acoustic nonlinear parameters is developed and manufactured on the basis of the effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected combination signal makes it possible to reconstruct the complete image of an object as a result of a single experiment using a small number of transducers, i.e., three radiators and one receiver. A mirror system is proposed, consisting of two coaxial conical acoustic mirrors that make it possible to transform the front of a wave from a cylindrical transducer into a homogeneous quasi-plane beam with a large width close to the real medical diagnostics requirements. Results of physical experiments are given.

Photon-phonon interaction on periodic structures

Features of the interaction between optical and acoustical waves with the same wavelength and multiple of the structural layer period, propagating simultaneously through periodic structures, are analyzed. The current state of and prospects for photon-phonon crystals being used for amplification, the generation of gigahertz acoustic waves, and other nonlinear optical and acoustic effects are examined.

Correlation between oscillations and translational motion of droplets induced by surface acoustic waves: towards low power actuators

Actuators based on SAW are versatile tools to achieve, atomization, jetting, oscillations, or inner mixing of droplets. Basically, acoustic waves are generated at the surface of a piezoelectric substrate by a transducer consisting of interdigitated fingers. The acoustic energy is then transmitted to the drop whose motion is induced by nonlinear acoustical effects (acoustic streaming, radiation pressure). However, an increase of the temperature occurs inside the drop due to the dissipation of acoustic energy. This could be harmful for the manipulation of biofluids. In this presentation, we will show that the amount of energy required to move or deform droplets can be drastically reduced by exciting them at their eigen frequencies. We will also exhibit and explain the correlation existing between the droplets amplitude of oscillation and their translation speed.

Numerical study on the evolution of the shock-accelerated SF6 interface: Influence of the interface shape

The early phases of the shock interaction process on two-dimensional interfaces with different shapes are numerically investigated in this study, which are closely related to the shock refraction and reflection, vorticity production and transport. The numerical method employs an adaptive unstructured quadrilateral mesh, which can capture the wave pattern and interface evolution very well. Simulations are carried out under the conditions of an incident shock Mach number of 1.2 and the light/heavy (air/SF6) interface. Five different shapes are considered in the simulations: rectangle, ellipse, diamond and two kinds of triangle. The results show that the interfacial shapes can influence the wave patterns particularly on the shape and evolution of refracted shock waves. The generation and the distribution of vorticity on the interfaces with five different shapes also have dissimilarities. The circulation deposition on five interfaces is quantitatively investigated and compared with theoretical model. A good agreement is found between the numerical results and the predictions by the theoretical model. Some characteristic scales of the interface are tracked. Under the influence of nonlinear-acoustic effect and vorticity effect, the interfaces present different evolution modes.

Vortex Shedding and its Nonlinear Acoustic Effect Occurring at a Slit

This paper presents a theoretical study of the nonlinear acoustic properties of a slit in a thin plate subjected to highintensity sound excitation. In the present model, the conventional discrete vortexmethod is employed to simulate the near-field two-dimensional unsteady flow in order to capture the mechanism of the sound–vortex interaction, which is in combination with a spanwise-averaged three-dimensional Green’s function method used to connect the nearfield flow quantities with the far-field sound pressure. This model is compared with an existing particle image velocimetry flow visualization and a direct numerical simulation model, showing good qualitative agreement. It is revealed that the oscillatory slitflow is dominated by a pair of spiral-like counter-rotating vorticesmoving away from the slit and eventually colliding into each other. Because of the vortical flow effect, increases in the sound pressure level result in significant increases in acoustic resistance and modest decreases in acoustic reactance. For the parametric range in this study, increases in the aspect ratio of the slit result in slight increases in acoustic resistance but unnegligible reductions in acoustic reactance. However, the influence of the aspect ratio on acoustic impedance tends to be less important when the sound pressure level exceeds a certain high value.

Investigation of Nonlinear Acoustic Effects in Two-Dimensional Open Cavity Flow

The aeroacoustic field generated by incompressible airflow over a two-dimensional open cavity is investigated. The main objective of this study is to determine the role of nonlinearity on this phenomenon. To this end the cavity aeroacoustics is investigated with both linearised and nonlinear approaches. Both free-air and confined cavity geometries are investigated and it is shown that nonlinearity does not affect the aeroacoustic field much. The solutions are verified for grid and computational domain independency. It is concluded from this study that nonlinear aeroacoustic phenomena are very weak in external incompressible flows and low-cost linearised formulations can easily be applied in such cases.